1. Field of the Invention
The present invention relates to a process for upgrading a liquid petroleum or chemical stream wherein said stream flows countercurrent to the flow of a treat gas, such as a hydrogen-containing gas, in at least one reaction zone. Injecting feed into one or more downstream reaction zones controls the temperature of at least one reaction zone.
2. Background of the Invention
There is a continuing need in the petroleum refining and chemical industries for catalyst and process technology that result in increase yields of desirable products and lower yields of undesirable components, especially those related to environmental concerns. One such process technology, hydroprocessing, has been subjected to increasing demands for improved heteroatom removal, aromatic saturation, and boiling point reduction. More active catalysts and improved reaction vessel designs are needed to meet these demands. Countercurrent hydroprocessing, where the liquid feedstream flows counter to upflowing treat gas, has the potential of meeting some of these demands because they offer certain advantages over co-current process where the liquid feedstream and treat gas flow co-currently. Countercurrent hydroprocessing is well known, but it has never reached its commercial potential, primarily because of flooding problems. Within a counter current flow reactor the up flowing treat gas becomes saturated with reaction products and lighter components of the feed. Typical reaction products of consequence are H2S, NH3, H2O, and light hydrocarbon products due to cracking, saturation, or heteroatom removal. These species increase the mass flux of the vapor phase thereby reducing the hydraulic capacity of a given diameter reactor; they also depress hydrogen partial pressure thereby reducing favorable reaction kinetics and thermodynamics. The condensable portions of these species present additional problems because as they move up the reactor into cooler or reduced treat gas (due to consumption) regimes they may condense increasing the down flowing liquid rate. This phenomenon can create a reflux loop within the reactor that can exceed the fresh feed rate. The refluxing is detrimental for two reasons: hydraulic capacity of the given reactor diameter is reduced and feed dilution results in less favorable reaction kinetics and thermodynamics.
A countercurrent process is disclosed in U.S. Pat. No. 3,147,210 that teaches a two-stage process for the hydroprocessing-hydrogenation of high boiling aromatic hydrocarbons. The feedstock is first subjected to catalytic hydroprocessing, preferably in co-current flow with a hydrogen-rich treat gas, then subjected to hydrogenation over a sulfur-sensitive noble metal hydrogenation catalyst countercurrent to the flow of a hydrogen-rich gas. U.S. Pat. Nos. 3,767,562 and 3,775,291 disclose a similar process for producing jet fuels, except the jet fuel is first hydrodesulfurized prior to two-stage hydrogenation. U.S. Pat. No. 5,183,556 also discloses a two-stage concurrent-countercurrent process for hydrofiningxe2x80x94hydrogenating aromatics in a diesel fuel stream. Any resulting vapor phase reaction products are swept upwards by the upward-flowing treat gas. Such vapor-phase reaction products may include relatively low boiling hydrocarbons and heteroatom components, such as H2S and NH3, as well as a heavy hydrocarbon tail gas. The heavier molecules in the vapor phase product of countercurrent hydroprocessing decrease its quality and make further hydroprocessing of the vapor phase product difficult.
Therefore, there still exists a need for improved countercurrent hydroprocessing reaction designs.
In accordance with the present invention there is provided a process for hydroprocessing a hydrocarbonaceous feedstream, which process comprises:
a) introducing a portion of said feedstream into a reaction vessel upstream from at least one reaction zone and passing said feedstream through two or more reaction zones operated at hydroprocessing conditions, wherein each reaction zone contains a bed of hydroprocessing catalyst;
b) introducing a hydrogen-containing treat gas at the bottom of said reaction vessel and passing it upward through at least one reaction zone countercurrent to the flow of liquid feedstream, thereby reacting with said feedstream in the presence of said hydroprocessing catalysts and resulting in a liquid phase product stream and a vapor phase product stream;
c) passing the liquid phase product out of the bottom of said reaction vessels;
d) removing the vapor phase product stream overhead of said reaction zones; and
e) wherein the temperature of one or more of said reaction zones is controlled by introducing a fraction of said feedstream upstream of said one or more reaction zones.
In a preferred embodiment of the present invention said temperature of said reaction vessel is controlled so that its temperature throughout the vessel is substantially isothermal.
In another preferred embodiment of the present invention there are two or more discrete feedstreams being fed to said process and the fraction used for temperature control is selected from the feedstream that is less difficult to hydroprocess.